Duplex ultrasonography of flow-mediated vasodilation (FMVD). The methods are as follows: Diameter of the brachial arterial (DBA) will be measured at multiple single time points under 4 states using a standard 7 MHz linear array transducer and a Hewlett-Packard system (Hewlett-Packard, Palo Alto, California). The four states include: at rest, during limb ischemia (created by inflating a blood pressure cuff placed distal to the brachial artery to supra-systemic pressures), in response to reactive hyperemia (with increased flow producing endothelium-dependent vasodilation). Measurements of these three states will then be repeated a second time. Finally, DBA will then be measured in (4) response to the administration of sublingual nitroglycerin. ADMA (asymmetric dimethylarginine) will be measured.